Stomach cancer is a malignant tumour in the stomach. In most cases it takes the form of an adenocarcinoma, a tumour that starts in the gland ducts within the stomach's mucous membrane. Stomach cancer is an uncommon form of cancer. It is twice as common in men than women and mainly occurs in older age. One of the first symptoms is stomach pain after a meal. In many cases stomach cancer is only detected at a late stage, which means prognoses can be poor.